The invention is concerned generally with so-called copying machines on a method in which the same subject matter, such as for example a document, is to be repeated in multiple copies. Specifically, the invention relates to such copying machines which utilize the electrostatic principles of imaging.
Two type of presently used electrostatic copying machines are in general use, one operating on the basis of xerography and the other on the basis of that which is known as electrofax. The first type of device provides an electrophotographic member in the form of an amorphous selenium drum based upon metal, the selenium surface being charged, exposed to an image, toned and the toned image transferred to a receptor which usually consists of ordinary paper. After transfer, the image is fused to the paper by a heat lamp. The second type of device provides an electrophotographic member in the form of a sheet of conductive paper carrying a coating of zinc oxide in a resin matrix, the coating being charged and exposed to projected image after which the entire electrophotographic member is moved through a liquid toner bath and fused in the latter step of which it is dried. The entire member becomes the copy of the document or article which was copied.
In both cases, when multiple copies are required, the apparatus recycles, passing through the functions of charge, expose and tone. In the case of the xerographic apparatus the electrophotographic member is caused to transfer the toned image to the receptor, the receptor is fused and dispensed into a bin, the member is cleaned and discharged and the mechanism starts all over again to charge, expose, tone, etc. In the case of the electrofax apparatus, obviously since the copy is the electrophotographic member itself, the machine must go through the complete cycle from the very beginning and through to the dispensing of the toned electrophotographic member for each copy.
Although this invention is concerned only with apparatus in which there is a transfer of toner effected, both types of electrophotographic members will be discussed as a group and contrasted with a different type of electrophotographic member.
The charge which is placed upon the surface of the known type of electrophotographic member as used in the xerographic and electrofax apparatus is selectively dissipated by the light which is projected onto the surface to form the latent image, leaving a latent image of primarily positive charges in the first case and negative charges in the second case. When toned by applying the fine particles of suitably polarized carbons, resins, etc. constituting modern toners, the toner particles neutralize or absorb the charges constituting the latent image. If these particles are removed mechanically, as by transfer to a receptor, substantially no charge of any consequence remains on the surface of the electrophotographic member. Either the toning neutralizes the charge or absorbs it, probably the former.
Using this type of electrophotographic member it is seen that known electrostatic copying machines must be complex and expensive, must have expensive, long duty cycles and must have slow throughputs. The rate of reproduction is limited by the length of a complete duty cycle.
The invention contemplates the use of an electrophotographic member in which the charge applied thereto is not believed to reside on the surface but is believed to exist at or below the surface without in any way adversely affecting the surface effects of the charge. Such an electrophotographic member is of the type which is disclosed in the copending application and includes a crystalline, dense, inorganic photoconductive coating of a thin film semiconductor material applied by sputtering to a suitable ohmic layer and substrate and used to make multiple copies as described below. Since there are believed to be no charges sitting on the surface, there can be no neutralization of such charges by toner particles applied thereto and thereafter removed therefrom. The charge and hence the latent image will obtain so long as the quality thereof does not deteriorate on account of the characteristic dark decay of the coating. Until this deterioration is obvious in copies produced, the latent image once formed can be toned and transferred repeatedly without the need for again charging and exposing so that many copies can be made quickly on only one exposure.
Advantages accrue because of the fact that one charge and exposure will provide a great many copies at high speed. The duty cycle during the copy making period of time consists only of toning and transferring.
Many other advantages will occur to those in this art as a result of consideration of the detailed description hereinafter.